What are credible and non-credible sources for a research paper?

Can someone brief me about the sources we can trust for citing in our research paper? Is it necessary that everything in print is reliable and trusted resource and everything online (such as blog posts, social media posts) is fraud or untrue? It is just a thought that many times occurred to me that if such social media posts, articles or e-journal articles are non-credible then why APA, MLA Handouts recommend the citation formats for them?